ipv4/igmp: Don't drop IGMP pkt with zeros src addr

Don't drop IGMP packets with a source address of all zeros which are
IGMP proxy reports. This is documented in Section 2.1.1 IGMP
Forwarding Rules of RFC 4541 IGMP and MLD Snooping Switches
Considerations.
Signed-off-by: Edward Chron <echron@arista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -1608,7 +1608,8 @@ int ip_mc_validate_source(struct sk_buff *skb, __be32 daddr, __be32 saddr,
 		return -EINVAL;
 
 	if (ipv4_is_zeronet(saddr)) {
-		if (!ipv4_is_local_multicast(daddr))
+		if (!ipv4_is_local_multicast(daddr) &&
+		    ip_hdr(skb)->protocol != IPPROTO_IGMP)
 			return -EINVAL;
 	} else {
 		err = fib_validate_source(skb, saddr, 0, tos, 0, dev,